Pulse amplifier



April 20, 1954 A. VAN WEEL 2,676,214

PULSE AMPLIFIER Filed Feb. 13, 1951 1.14411 1 wmmn "Y A T 50 a 1].;INVENTOR 45.3.

' ADE BERT yAN EEL AGENT Patented Apr. 20, 1954 I PULSE AMPLIFIERAdelbert van Weel, Eindhoven, Netherlands, as-

signor to Hartford National Bank and Trust Company, Hartford, Conn., astrustee Application February 13, 1951, Serial No. 210714 Claimspriority, application Netherlands March 8, 1950 14 Claims. 1

This invention relates to pulse amplifiers for the amplification ofsignal pulses modulated on a carrier wave and may be used with advantagein pulse radar apparatus, transceiving devices for pulse modulation,relay stations (repeater sta- 'tions) for beam transmittercommunications, and

the like. i carrier wave by amplitude modulation or by fre- The pulsesmay be modulated on the quency modulation.

7 The invention has for its object to realize the following combinedadvantages in selective pulse amplifiers comprising at least oneamplifying tube and at least one oscillatory circuit tuned to thecarrier wave:

-' l; A substantial decrease in the amount of signal distortion;

2. A highly effective discrimination between "signal pulsesand'interference pulses;

3. Undistorted transmission of the trailing edge 1 of the signal pulses;

' 4. A substantially constant amplifier gain for hoth'signal andinterference pulses.

According to the invention, the oscillatory circuit is connected, to avariable damping'resistance, with both this resistance and the mutualconductance of the amplifying tube being controlled by control-pulsesproduced as a function of the signal pulses to be amplifiedin suchfashion J that each time a signal pulse is supplied to the oscillatorycircuit, the damping of the oscillatory circuit and the mutualconductance of the amplifying tube are increased. The increased dampingwidens the bandwidth of the amplifier but attenuates the signal; theincreasing mutual conductance increases the amplifier gain suincientlyto compensate for the signal attenuation.

The control-pulses may be obtained by means of. amplitude detection ofthe signal pulses modulated on a carrier wave. As an alternative, thecontrol-pulses may be taken from a local pulse generator. I l

In order that the invention and its advantages may be more clearly.understood and readily carried into efiect, they will now be describedmorefully with reference to the accompanying drawing. I 1 Fig. 1schematically shows'a pulse receiver comprising a pulse amplifieraccording to the invention; I

Fig. 2 schematically shows a pulse receiver including in modified formthe pulse amplifier shown in Fig. 1, and I Fig. 3, curves a, b, c, and dgraphically show the output voltage yielded at various stages in I I g 7-66 When the amplitude of is detected signal pulse the pulse amplifiershownin Fig.1.

In the pulse receiver shown in Fig. 1, signal pulses modulated on acarrier wave are received through an aerial I. The aerial I is connectedto a receiving part 2, comprising a mixer having a local oscillator 3and, if necessary, several intermediate-frequency amplifiers connectedin cascade; the bandwidth of the said stages is sufliciently large toensure a faithful amplification of the incoming pulses.

The pulses emanating from the receiver part 2 w and modulated on theintermediate frequency are fed through a selective input circuit 4, 5 tothe control-grid of a pentode 6 of the indirectlyheated type and'used asa voltage amplifier. The

I pentode 6 has a selective output circuit 1, 8 and a grounded cathoderesistance 9, which is shunted V bya by-ip'ass condenser I0.

For further voltage amplification, the output circuit I, 8 of "thepentode 6 is connected to the control-grid of a second pentode I I,connected as m a voltage amplifier, in the same fashion as pentode 6.The pentode II comprises a cathode resistance I3, shunted by a by-passcondenser I2, and an output circuit I4, I5 tuned to the intermediatefrequency. I

The bandwidth of these two intermediate-frequency amplifiers is normallyinsufiicient to ensure a faithful amplification of the signal pulses.

The receiving cascade furthermore comprises an amplitude detector ofknown type, which de- 30 tects the signal pulses which are transmittedby the modulated intermediate frequency oscillations. I The amplitudedetector comprises a diode H, which is shunted by a resistance I6, thecathode'of this diode being connected via a detection condenser I8 tothe output'circuit I4, I5. The diode I1 constitutes a material load forthe output circuit I4, I5, so that the selectivity of the circuit I4, I5is appreciably reduced. In order to suppress-undesired oscillations, thedetected pulses occurring across the output resistance I6 of the diodedetector are supplied to a low bandpass filter comprising a seriesresistance I9 and a parallel condenser 20. r 1

The'output circuit of the low band-pass filter is connected to thecontrol-grid of a triode 2|, used as a threshold device and cut off innormal use, having an anode resistance 22 and a cathode resistance 23.The negative grid-bias required to cut off the triode 2| is taken from apotentiometer 25 included between ground and a negative terminal 24 of agrid-bias battery and connected through resistances I5 and I9 to thecontrol-grid of the triode 2|.

. the selective oscillatory capacitors 33 resistance 23 of thetriode 2|.

. 6 and H and rendering diodes 3|v and 32 thereby exceeds the thresholdvalue of the triode 2|, a voltage pulse of negative polarity is producedacross the anode resistance 22. The voltage pulses thus obtained afterpassing the threshold are fed through a device 26, which will bedescribed hereinafter, to a pulse-demodulator 21, the output circuit ofwhich is connected to a reproducing device 28.

In the pulse receiver so far described, a substantial decrease ininterference is obtained because the selectivity of the two voltageamplifier circuits is such that the interference spectrum bandwidth isdecreased. However, this decrease in interference is offset by thedisadvantages that, due to the slight damping of the selective circuitsemployed in the pulse receiver, the leading and trailing edges ofincoming pulses create transients with appreciable time constants. Ithas been found in practice that a further reduction of the bandwidthbeyond a certain predetermined value willbe of no value indecreasing.interference.

According. to theinvention, the selective oscillatory circuits 4, 5 andI, 8 are each connected to a variable damping resistance, eachresistance and the mutualconductance of each of amplifying tubes 6 andI] being included in the pulse amplifier being controlled bycontrol-pulses produced' as a function of the .signal pulses, so thatwhen a signal pulse is supplied tothese circuits, both .thedamping andthe. mutual conductance are increased.

The additional .damping'of the oscillatory circuits permits a faithfulamplification of the signal pulses, whilst the increase in mutualconductance is. utilized to counteract thereduction in amplificationproduced by. said damping.

In the pulse amplifier. shown in Fig. 1, each of circuits 4, 5 and l, 8has connected to it in parallel a variable damping resistance,constituted by the series-combination of a resistance 29 and 30respectively and adiode 3| and 32 respectively, the anodes beingconnectedto the control grids of the tubes 6 and. The diodes 3|.and 32have then cathodes connected through the series resistances 29 and -30to tappings on the cathode resistances 9 and I3,

these tappings being grounded through-by-pass and 34, sothatbias-voltages are developed which; normally .render.-both diodesnon-conducting.

The control-pulses are taken from the cathode and supplied through acontrol-voltage channel 35 to the control-grids of the amplifying tubes6 and Each time when a-signalpulse occurs, the cathoderesistance 23 ofthe triode 2| has-produced across it acontrol-pulse'of positivepolarity, these control-pulses producing; simultaneously an increase inmutual'conductance of the amplifying tubes conductivethe damping of theassociated oscillatory circuits.

Thev operation of the circuit-arrangement will now be set out in detail.

Uponv initiation of a signal pulse, .the voltage across the oscillatorycircuit l4, 15 increases com-* paratively slowly as afunction of theselectivity of the oscillatory-circuits4, 5 and 1., 8. -When,

.- after a certain time, AT, the voltage acrossthe oscillatory circuitl4, l5 exceeds a certain thresholdvalue, which is approximately equal tothe sum ofv the threshold voltage of theatriode 2| and the cut-offvoltage of the. diodes 3|, .32,..the

damping of the oscillatory circuits 4, 5 -and I, 8

is increased asdiodes 3| andjz-becomecona increasing the-damping 4release of the. 'diodes 3! and 32,? the bandwidth fication, whereas-withfaithful amplification of the incoming signal A. higher damping of ofthe circuits 4, 5 and l, 8 approximately corresponds with the bandwidthof the amplifyin the preceding receiver part 2. the circuits 4, 5 and 1,8 only produces an unnecessary reduction in ampliunduly small damping astages included in pulses is not guaranteed.

Upon occurrence, of a signal pulse, the damping of the oscillatorycircuits 4, 5 and T, 8 and the mutual conductance of the associatedtubes 6 and H are preferably increased at least to such width of theoscillatory circuits.

with optimum. damping. 1 .Each spectrum, com- .anextent that theamplification of the pulse amplifier remains substantially constant.Then,

. the pulse amplifier .ihas. the featurethat with constantamplification, the bandwidth is;abruptly increased when a signalpulseoccurs.

It. should. benoted here that. the. frequency spectrum of the signalpulses occurring across the oscillatory circuit I4, |5..varies with the.bandponent of the detected signal.pulsescorresponds totwo side-bandfrequencies ofthe intermediatefrequencywcarrier oscillation: modulated.by the ,signal pulses.

trum of the detected. signal pulses. is approxi- The width of thefrequencyspecmately equal to half the valueof the..optimum-band-width-.of, theoscillatory-circuits,4; 5.. and

In order to -obtain a.satisfactoryreduction of interference; in practiceit has-been found particularly advantageous to supply the-contro1-pulses obtained by detectionof ,the. signal pulses through alow-massfilter to. the -amplifying tubes 6 and l l, the passage of.thelow pass, filter, being .at the most, equal to half-the value. ofthe-optimum bandwidth of theaoscillatorycircuits 4, 5

I and 1-, 8. z-In the embodiment shown, the low pass filter isconstituted. by... a -series resistance -36and a-paral1el-condensen31.Theoutput-of the lowpass filter is connected -th-r;ough;; high-"time-lag of.

frequency coils 38 and 39 respectively tothe-control-grids of the.amplifying tubes Band. I the coils 38 and 3,9. servingto decouplethe-oscillatory circuits 4, 5and i, B.

,1 For an undistortedtransfer of the-trailing edge of the. signalpulses; the control-pulsesihave-=to remain operative :untik termination:of thersignal pulses to==1be= amplified. K For this purpose, ,the

the control-pulses introduced into I the control-voltage{channelz35nby,the 10W; pass filter 3B,:3'I -andthe high-frequency:coils';38;.39

-- and the; assoc-iated-zcut-oif condensersr lfl, 4|; has

to exceed the duration of the timeperiod over which .the,=trailing edgeof the. signalvpulses is suppliedto the'pulseamplifier.

' .The operation of theapulsei-amplifier described .willnow .-be.explained fmore fully with reference graphs shown in Figs. 3a, b, c, d.m Fig. 3am'ShOWSrtWo; signalpulses- 42 and 42 suppliedfrom the outputof-the receiverpart 2 and also shows three intermediate interferencepulses, of, carrier-wave" frequency 43,44, 45,. this frequencycorresponding to the tuning frequency of the oscillatory circuits 4-, 5.and- I, 8.. As com- .pared with-the iamplitude andaduration. of thesignal pulses 42 and-:42 the amplitude and duration of interferencepulse 43 are respectively larger and smaller, while the amplitude andduration of interference pulse 44 are both respectively smaller, and theamplitude and duration of interference pulse 45 are respectively smallerand larger. a

Fig. 3b shows the amplified signal pulses 46, 46' occurring across theoscillatory circuit I4, l5 and interference pulses 41, 48, 49.

When the signal pulses are first applied, transients are produced, asindicated by curves 50 and 50. Each time after a certain time-lag AT,the damping of the oscillatory circuits 4, 5 and 1, 8 is suddenlyincreased; thereafter the signal pulses are amplified with optimumbandwidth of the pulse amplifier. The variation of the amplified signalpulses 46, 46' then is as shown in the figure.

The pulse amplifier shown provides a highly effective discrimination induration and amplitude between signal andinterference pulses.

Interference pulses of comparatively short duration as indicated by d?and 44 in Fig. 3a, produce transient excitation of the selectivecircuits 4, 5 and], 8. Before the amplitude of the oscillations acrossthe circuits4, 5 and I, 8 has reached its final value, the pulsesdil and44 are terminated, with the result that the amplificaton factor forsuchinterference pulses is materially lower than for the signal pulses.Consequently, short interference pulses occur in an attenuated formacross the output circuit id, [5 of the pulse amplifier. The pulsatorycontrol-voltage obtained after detection and passage of suchinterference pulses through the threshold is usually insufiicient torelease the damping diodes 3i and 32.

Upon amplification of interference pulses of'a comparatively longduration, as indicated by 45 in- Fig. 3a, the amplitude of theoscillations across the circuits 4, 5 and l, 3 substantially reaches itsfinal value. Such interference pulses of small amplitude, however, donot bring about an. increase in damping of the oscillatory'circuits 4, 5and 1, 8. It is important that the slopes of the leading and trailingedges of the interference pulses amplified at high selectivity, aindicated by 4?, 48 and 49, should be materially smaller than those ofthe signal pulses 4'6, 4'8. 1

Fig. 3c shows the pulsatory output voltages across the anode resistance22 of the triode 2!, operating as a threshold device. The thresholdvoltage i indicated by a horizontal line 53. Only those pulse parts ofthe signal pulses'52, 52' and of the interference pulses 53, 54 whichexceed the threshold voltage are allowed to pass, whereas theinterference pulse 55 is completely suppressed.

In order to ensure optimum freedom from interference the anode of thetriode used as a threshold device is connected to a difierentiatingnetwork built up from a series condenser -55 and a parallel resistance51. The output circuit of than that of the differentiated interferencepulses 68, 6|.

The trailing edge of the incoming signal pulses is amplifiedsubstantially without distortion. Consequently, for pulse position,pulse-frequency modulation and the like, the voltage pulses obtained bydifferentiation and those coinciding with the trailing edge of thesignal pulses may beutilized with advantage in the further part of theapparatus.

In the pulse amplifier shown, the output resistance 51 of thedifferentiating network is con-' nected for this purpose to the anode ofa diode '52, which i usually cut-off by a bias voltage and the cathodeof which is grounded through a resistance 83. The cut-01f voltage isobtained by connecting the resistance 57 to a potentiometer 84 includedbetween the negative terminal 24 of the grid-bias battery and ground andthe tapping of which is grounded through a smoothing condenser 65. Thecut-off voltage of the diode is indicated by a horizontal line in Fig.311.

If a transient positive voltage pulse coinciding with the trailing edgeof the signal pulse exceeds the cut-off voltage of the diode 62, thediode is thus transiently released and a short voltage pulse i suppliedto the pulse demodulator 2?. The differentiated interference pulses 6G,5! are completely suppressed byth threshold diode =52. If necessary,thethreshold device is followed by a relaxation generator for producingrenewed pulses. Such pulse regenerators are particularly important inrelay stations.

When using a pulse amplifier according to the invention, a particularlyfavourable efficiency may be obtained. The amplifying tubes it and H arepreferably adjusted in normal use to low anode current. The amplitude ofthe controlpulses is so chosen, with respect to the supply voltages ofthe tubes, that the control-pulses each time produce an overload of theamplifying tubes by the signal pulses to be amplified. With theadjustment of the tubes as described, the excess the differentiatingnetwork comprises an additional threshold device. Each time at thebeginning and at the endof a pulse, the resistance 5?! of thedifferentiating network has produced across it transient voltage pulsesof negative or positive polarity, the-amplitude ofwhich is de terminedby the slope of the leading and trailing edge of the pulses supplied tothe differentiating network. I

The variation of the differentiated output voltage of the triode 2! isshown in Fig. 3d. It is evident from the figure that the amplitude ofthe voltage pulses 58, 55 and 58 vand'59' coinciding with the beginningand. the end of the signal Pulses see 5? resist vityunsi ht r a of thepermissible dissipation produced by transient overload should do no harmto the amplifying circuit.

It is not necessary to supply the control-pulses to the control-grids ofthe amplifying tubes. As an alternative; the control-pulses may besupplied to other electrodes of the amplifying tubes, for example to thecathode, screen-grid or anode or simultaneously to several electrodes.

The variable damping resistances may be of different construction, andmay, for example, be

* constituted by a rectifier cell or an amplifying tube.

"Fig. 2 shows 'a pulse receiver comprising a variant of the pulseamplifier shown in Fig. 1. Similar elements are designated by the samereference numerals.

' The pulse amplifier shown in Fig. 2 comprises the cascade connectionof two grounded grid amplifying stages. Each of the amplifying stagescomprises a triode 5! and 63 respectively, the cathodes of which aregrounded through tappings ofv selective input circuits 69, iii and H, #2respectively and cathode resistances l5 and 76 respectively, shunted bysmoothing capacitors l3 and 14 respectively. The output circuit of thetriode I31 is constituted by the oscillatory circuit ll, i2 whereas theoutput circuit of the triode '63 comprises the parallel combination ofan oscillatory circuit l1, l8 and a damping resistance 79. v For thedetection of the oscillations occurring assume across the dampedoscillatory circuit ll-19; the latter is connected to the anode of adiode 80', connected as a-deteetor and the cathode of which is groundedthrough a detection resistance s2 shunted by a condenser 81. Thedetected oscillations are supplied through :a series resistance 83*tothecontrol-grid of a triod' 84, the anode circuit of which includes acouplingcondenser. 85 and a transformer- 86, provided with ahighirequency iron core and operating as a differentiating network.Across the secondary winding, which. is shunted by a .rectifier Bl,voltage pulses of negative and positive polarity occur each time at thebeginning and at the end of a signal pulse, the negative voltage pulses.being suppressed by the rectifier cell 81. The positive voltage pulsescoinciding with the end of the signal pulsesare supplied to a pulsedemodulator 21 through a threshold device 62-64, the operation of whichis described with reference to Fi l.

Across the cathode :of the-detector diode 80 are produced control-pulsesof positive polarity, which are supplied to the control-voltage channel35* including the low pass-filter 36, 31. The output circuit of the lowpass filter 36, 31 is connected through decoupling resistances 88, 89 tothe control-grids of the amplifying tubes 67, '68, these control-gridsbeingconnected to ground through coupling condensers 90, 9|.

In a grounded grid amplifier, the input impedance is approximatelyinversely proportional to the mutual conductance of the amplifying tube.The amplifying stages shown in Fig. 2 thus operate as :if a dampingresistance varying with the mutual conductance of the amplifying tubewere connected-between the tapping of the oscillatory circuits 59, itand-ll, I2 andground, in a manner such that the damping of theoscillatory circuitsfiii, iii and H, '52 increases with the mutualconductance of the tubes. 61, 58 in the arrangement shown thus performat the same time the function of variable damping resistances.

The amplitud of the pulse-shaped controlvoltage corresponding tointerference pulses of the type shown in Fig. 3a is materially smallerthan the amplitude of the control-pulses produced as: a function of" thesignal pulses. The slopes-of the leading and trailing edges: of theamplified signal pulses then appreciably exceed those of the amplifiedinterference'pulses.

The pulse amplifier shown differs from that describedwith-reference toFig. l in that here the bandwidth increasescontinuously with thecontrol-voltage.

In the amplifying arrangements shown, the control-pulses are obtaineddirectly by detection of the. signal pulses. For producing thecontrolpulses*,,use may alternatively be made of a local pulsegenerator, which is synchronized bythe incoming signal pulses. in aknown manner, for producing release pulses.

ApartTfrom the pulse amplifiers shown, other constructions are possible,more particularly amplifying arrangements comprising an amplifying tubehaving a secondary-emission electrode. In such tubes material variationsin mutual conductance are possible, for example, by a factor l0.

More particularly in pulse-amplifying arrangements comprisingsecondary-emission tubes, it is possible to insurethat, upon occurrenceof signal pulses, the amplification of the pulse amplifier as a wholeis'materially increased by the controlpulses.

As mentioned before, the arrangement shown The amplifying tubesmayifurthermorerbe utilizedi for the amplification of. 'pulses'modulated on; a carrier: wa ve by -fre-:- quency modulation; "lihepulse. amplifier shown essentially-remains the same, only the amplitudedetector is required to be replaced by a frequency detector-.-

What Iclaimis:

1. Apparatus for i amplifying signal. pulses appearing asa modulationcomponent on a carrier wave,-; said apparatus 1 comprising an.amplifying stage including. an electron .discharge tube, means tovapplysaid signal pulses as an: input to said stage;-anoscillatorycircuit coupled to said stage for determining the band passcharacteristic of said-stage, a variable damping impedance coupledtosaidoscillatory circuitfor varying the'quality thereof, means coupledto the output of said amplifying stageand responsive to the amplifiedsignal-pulses therein for producing direct-current control pulses as afunction of said signal pulses, and means to supply the direct currentcontrol pulses to: said tube and said impedance for simultaneouslyincreasing the mutual conductance of said discharge tube and decreasingthe value of said dampingimpedance, thereby increasing the damping ofsaid oscillatory circuit.

2. A circuit arrangement, as set forth in claim 1, wherein the increasein the damping, of the oscillatory circuit and the increase in themutual conductance of the electron discharge tube are such that theamplification of said circuit-an rangement remains constant.

3. A circuit arrangement, as setforth in claim 1, wherein the controlpulse causes the amplifying tube to saturate when the signal pulse isapplied.

4. A circuit arrangement, as set forth in claim 1, wherein the controlpulse becomes operative only after the signal pulse has reached acertain threshold value.

5. Apparatus, as set forth in claim 1, wherein said electron dischargetube in said amplifying stage includes a cathode, a grid and an anode,and circuits therefor, said cathode being connected to a highefrequeneyground, and wherein said oscillatory circuit is-included in. the anodecircuit of said stage and said variable damping impedance isv connectedin shunt relation with said oscillatory circuit.

6. A.circuit arrangement, as: setiorth in claim 5, wherein the variabledamping impedance is connected in parallel with the oscillatory circuitsaid variable impedance constituted by the series combination of aresistance and a diode, said diode normally being renderednon-conductive by a bias voltage, said diode becoming conductive onlywhen control: pulses are present.

7. A circuit arrangement, as set forth in claim 6, wherein the anode ofthediode is coupled to the control grid of the electron discharge tubeand wherein the control pulses possess positive polarity and:wherein-said pulses are supplied to said control grid and the anode ofsaid diode.

8. Apparatus for amplifying signal pulses appearing asa-modulationcomponent on a carrier wave, said apparatus comprising an amplifyingstage including an electron discharge tube, means to apply said signalpulses as an input to said stage, an oscillatory circuit coupled to saidstage for determining the band-pass characteristic of said stage, avariable damping impedance coupled to said oscillatory circuit forvarying the quality thereof, means-coupled to the output of saidamplifying stageand responsive" to the amplified signal pulses thereinfor producing'direct-current control pulses as a function of said signalpulses, and means to supply the direct-current control pulses to saidtube and said impedance, comp-rising a direct-current control pulsechannel including a low-pass filter which is connected to saidamplifying stage, for simultaneously increasing the mutual conductanceof said discharge tube and decreasing the value of said dampingimpedance.

9. Apparatus for amplifying signal pulses appearing as a modulationcomponent on a carrier wave, said apparatus comprising an amplifyingstage including an electron discharge tube having a cathode, a grid andan anode, and circuits therefor, means grounding said grid for signalfrequencies, means to apply said signal pulses as an input to saidstage, said tube and circuits including an oscillatory circuit includedin the cathode circuit of said stage which determines the band passcharacteristic of said stage, a variable damping impedance coupled tosaid oscillatory circuit for varying the quality thereof, means coupledto the output of said amplifying stage for deriving therefrom controlpulses of positive polarity, and means for supplying said control pulsesof positive polarity to said grid whenever a signal pulse occurs andthereby simultaneously increasing the mutual conductance of saiddischarge tube and decreasing the value of said damping impedance, whena control pulse is present.

10. A circuit arrangement, as set forth in claim 9, wherein the meanscoupled to the output of the amplifying stage for deriving therefromcontrol pulses comprises a signal pulse detector having adifferentiating network included in the output circuit.

11. A circuit arrangement, as set forth in claim 10, wherein the outputcircuit of the diilerentiating network comprises a threshold device.

12. A circuit arrangement, as set forth in claim 11, wherein thethreshold device only responds to voltage pulses which are obtained bydifferentiation and which correspond with the trailing edges of theincoming signal pulses.

13. A circuit arrangement, as set forth in claim 8, wherein the range ofthe low pass filter is at the most equal to half the band width of theoscillatory circuit with optimum damping.

14. A circuit arrangement, as set forth in claim 8, wherein the low passfilter has a time constant that enables the control pulse to be appliedover a greater time interval than the time of passage of the trailingedge of the signal pulse.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,066,333 Caruthers Jan. 5, 1937 2,236,690 Mathes Apr. 1, 19412,447,248 Harris Aug. 17, 1948

